Ducati Monster Forum

My bike goes in next week to get the PCIII setup on the dyno.  My mods at this point include a full system with a Leo Vince can and Marving mid pipe, K and N and the PCIII.  I have also picked up a second airbox to modify.

I have heard from various sources opening up the airbox may not increase performance at all as the volume of the air box is an integral part of the induction system.  I could care less if the bike is louder, I just want it to run as good as I can get it to and coax every ft-lb/hp I can out of it.

Does anyone have any experience with this?  I have a second air box I am willing to cut up, but if it isn't going to make a difference than I am not going to.  One of my other concerns is getting water in the air box from a fully open DP style air box.

Thoughts?

KNG

Killer, the induction is a complex topic.  I looked into that a few years ago and there are a few things you can look into and do.  I calculated the airflow needed by the engine at various rpms and found that north of 6K, the air flows supersonic in the duct by the Helmholtz resonator.  (My bike is an '03 M800S, your airbox may be different.)  This chokes the mass of air into the engine and causes turbulence losses on the intake. 

Slowing the intake velocity will increase the air pressure, which you want.   You can do this by increasing the inlet duct area.  I found around 90 cm2 would keep the intake mach number around .1 to 6K rmps, then .2 until the limiter kicks in.  I ended up cutting a duct in the rear of the airbox lid so the areas (front and rear duct) would match the airflow velocity for my engine.   I had posted pictures and data a while back when the other website was our hangout.  Don't know where they may be now.

I ride in the rain, so I don't go for the K&N open lid types.     

BC

Battlecry, can you detail out those calculations?

+1 I like to see that as well.

umm...
my aftermarket exhaust flows more
so i made my intake flow more (cut the lid open)
my mechanic made the gas flow more
my bike goes faster now
i ride in wet weather so i got a K&N filter

it all works good. yup

Killer:

Get the volume of air flowed by the engine per revolution and create a table with rpm to see how much air volume it gulps in.

Divide by the inlet area and you get intake velocity at the front duct.  Use an estimated intake air temperature to calculate the speed of sound and divide that velocity by the speed of sound and you get the mach number.  This is rough, but as you approach M=1, a shock wave will form at the area of constriction inside the duct.  The air conditions after the shock wave are interesting but this is a motorcycle, not a jet, so it is probably better increase the intake pressure by slowing the air velocity down, which means sucking it from an area of higher pressure (slower air).  You find that in the air pocket around and behind the engine.  Unfortunately, that air is hot, ideally, you want it as cold as you can get it. 

The Ducati engineers molded a Helmholtz resonator in the intake duct.  That is the small chamber under the intake duct.   Air flowing through the front duct flows over a hole in the chamber and excites the air to vibrate at a tuned frequency.  It is supposed to quiet the intake down.  Which may have some value. 

What I found interesting about it (the resonator) was that if you have a rear duct, it helps direct the flow so more of the rear duct air flows towards the front of the air filter.  The air coming in from the front duct sticks to the curvature of the resonator using the Coanda effect, then the air coming in through the rear duct flows around it towards the front.  You will find the size of the airbox lid fits very well in the intake for shop air cleaner.  That helps to visualize what is going on.   

I'm pretty happy with my setup, but note that I removed the sound pad under the tank and I'm using a small battery so more air can flow from the rear.

This is what the rear duct looks like:

(http://www.geologics.com/images/airbox_duct.gif)

These are the numbers I used:

(http://www.geologics.com/images/airbox_numbers.gif)

Much of the increase in performance you get from a DP kit is from taking the top off the airbox. If the Ducati factory folks could have, they would have shipped it that way and with a proper exhaust and fuel map too. 

Of course, you gotta get more fuel, and the PCM will give you that if you don't have an O2 sensor in the exhaust.  If it's an O2 sensor model, it won't fuel well as the PCM doesn't play well with the stock ECU's programing.  To be "perfect" you'd have to reflash the OEM ECU or get a DP ECU in which case you don't need the PCM anyway.  Confusing.

Your exhaust complete the picture if there's no DB killers. ;D  You should get several HP increase if all goes well.

Don't worry about the airbox and water, it's not a problem.  I ride in driving downpours much more often than I'd like with no ill effects.

LA

Isnt it the area under the filter? not the ecclosed area? hence the airlid just restricts?

You dropped a decimal point on your calculation for meters/sec.

Your line for Front M/S should start at 4.8, not 48.
Front M/S would be 43 meters/sec ( about 95 mph)

So Front Mach would peak out at 0.13 at 9000 rpm.

You're right, Speeddog!  Thanks man.